Effects of Co-Application of Diammonium Phosphate Fertilizer with Microbial Inoculant on Soil Nitrogen Levels and Alfalfa Growth Performance in Saline-Alkali Soil

Soil salinization leads to soil nutrient loss and decreased crop yield. This research aims to determine the optimal reduction rate of diammonium phosphate (DAP) and suitable microbial inoculant for alfalfa cultivation and nitrogen-level improvement in saline-alkali land. The experiment consisted of a factorial arrangement of three DAP fertilizer levels (X1, 60%; X2, 70%; and X3, 80%) and four microbial inoculants (Y1, rhizobial inoculant; Y2, phosphate-solubilizing microbial inoculant; Y3, composite microbial inoculant; and Y4, control) in a split-plot design. The results indicated that DAP fertilizer, microbial inoculant, and their interaction significantly affected (p < 0.05) forage yield, crude protein, available nitrogen (N), and enzyme activities. Under 80% DAP fertilizer combined with the composite microbial inoculant, forage yield, plant height, soil urease (S-UE), and ammonium nitrogen (NH4+-N) reached maximum values of 17.1 t ha−1, 65.7 cm, 292.3 μg d−1 g−1, and 3.1 mg kg−1, respectively. However, the soil total nitrogen (TN) significantly increased at the 60% DAP fertilizer application rate (p < 0.05). Overall, this study demonstrates that co-application of DAP fertilizer with compound microbial inoculant delivers a green, science-based fertilization approach for improving nitrogen levels and alfalfa cultivation in saline-alkali soils.